The present invention relates to a solid-state jointing technology of different kinds of metal members.
A frictional joining method using a frictional heat has been proposed as a joining method of different kinds of metal members (Japanese Patent Laid-Open Publication No. 2006-21217, US Patent Application Publication No. 2005/0035180). Herein, the metal members to be joined, and a rotating tool is pressed against one of the metal members. By pressing of the rotating tool, the metal member is softened and made in a plastic flow state, whereby the metal members are joined in a solid state.
This frictional joining method is advantageous to joining for different kinds of metal members. Namely, it is generally difficult to uniformly melt different kinds of metal members that have different physical properties, such as melting point or thermal conductivity, in a melting-joining method like an arch welding. Also, a fragile metal compound is generally generated between these metal members, so a sufficient strength may not be obtained. According to the frictional joining method, however, this generation of the metal compound may be prevented, thereby providing a properly strong joining. The above-described Japanese Patent Laid-Open Publication discloses the frictional joining method, in which the metal member that the rotating tool contacts is heated in advance, thereby the metal member may be softened to prevent the rotating tool from being worn away.
Meanwhile, there generally exists an oxidation film on a surface of a metal member. This oxidation film would block a proper direct contact of respective new uncovered surfaces of the metal members in the frictional joining method. Thereby, the joining strength would deteriorate. Herein, the oxidation film generated on the surface of the metal member, against which the rotating tool is pressed, may be destroyed by the plastic flow of this metal member. However, the oxidation film that is generated on the surface of another metal member, against which the rotating tool is not pressed, may remain without destruction. The above-described US Patent Application Publication discloses the joining method, in which a metal plating is provided on the surface of the metal member in advance in order to block the generation of the oxidation film, and this metal plating is configured to be pushed out from a joining portion of the metal members by a pressing force of the rotating tool, or to be taken into the metal member pressed by the rotating tool, thereby providing the proper direct contact of respective new uncovered surfaces of the metal members.
In this frictional joining method disclosed in the US Patent Application Publication, it may be desirable that the metal plating is properly pushed out from the joining portion of the metal members by the pressing force of the rotating tool, so that the respective new uncovered surfaces of the metal members can contact each other in a properly wide range. However, there exists metal members that have their different physical properties, and therefore the degree of the metal plating pushed out defers depending on the kind of the metal members.